Laser systems of the type noted above can exhibit bandwidth transients in response to duty cycle changes, whether of the single chamber or multi-chamber configuration, e.g., master oscillator/power amplifier (“MOPA”) excimer laser systems, e.g., of the ArF or KrF variety. The transients are believed by applicant to be the result of thermally induced changes to laser light pulses in the laser system optical train, e.g., cylindrical wavefront deformation in the line narrowing module (“LNM”) optics, optical coatings, and/or surrounding purge gas.
According to aspects of an embodiment of the present invention applicant proposes an apparatus and method for compensating for bandwidth and center wavelength transients with an actuated bandwidth control device (“BCD”), e.g., a mechanism for changing the curvature of the wavelength selection optical element, e.g., an eschelle grating, or a similar actuated optical element that can, e.g., introduce a cylindrical wavefront deformation on demand based, e.g., upon a feed forward controller algorithm.